Language-wise, most Austro-Hungarians are going to be multilingual. Once universal secondary education is achieved, all graduates are likely to be at minimum fluent in their home language/dialect and either Standard German or Hungarian. Germans and Hungarians are likely to learn one another's languages for fairly pragmatic reasons (Hungarians are advantaged by being able to talk to the other half of the Empire and also most of Central Europe, Germanophones are advantaged by being able to speak the other main language of the Empire, particularly if they want to work in the army or civil service).
A notable feature of primary education in OTL Austria-Hungary is that it was conducted in local languages - not out of any commitment to cultural diversity, but because there just weren't enough literate people at the time the system was being set up who could speak both German/Hungarian and the local language - this lead to the development of standard orthographies and literary forms for small languages like Slovenian (at the time usually called Carinthian) and Friulian in the late 18th/early 19th Centuries. By the 20th Century, this was too established to be easily shifted to German/Hungarian-only primary education.
Pragmatically, the individual components of this federalised Austria-Hungary are likely to also have official languages that are taught in schools, likely before German/Hungarian. At minimum, I'd expect Czech in Bohemia-Moravia, Polish and Ruthenian(Ukrainian) in Galicia-Lodomeria, Croatian in Croatia and Dalmatia (and possibly Istria and Bosnia), Serbian in the Banate (and possibly Bosnia), Romanian in Transylvania, Friulian in Friuli/Gorizia, Italian (or Venetan if they're trying to break links to Italy) in Trieste and Dalmatia, Slovak in OTL Slovakia (sooner or later Budapest is going to have to either grant it some degree of autonomy or else Vienna will split it off). Might also have some smaller groups like Rusyns, Romani, Aromanians etc get lucky and get some degree of official recognition.
Most likely, you end up with a system of mother-tongue education in primary school, National-language education in middle school, and German/Hungarian education in high school and university. The average citizen would be at least bilingual (Germans and Hungarians are likely to have the smallest linguistic repetoirs), while being able to speak 3 or 4 languages would be fairly unremarkable. Common foreign languages that might be taught as elective subjects could include Italian, Russian, French, English and Turkish, as well as the other national languages of the Empire.
(Gods, the Reichstag is likely to be as reliant on interpreters as the European Parliament is).
A notable feature of primary education in OTL Austria-Hungary is that it was conducted in local languages - not out of any commitment to cultural diversity, but because there just weren't enough literate people at the time the system was being set up who could speak both German/Hungarian and the local language - this lead to the development of standard orthographies and literary forms for small languages like Slovenian (at the time usually called Carinthian) and Friulian in the late 18th/early 19th Centuries. By the 20th Century, this was too established to be easily shifted to German/Hungarian-only primary education.
Pragmatically, the individual components of this federalised Austria-Hungary are likely to also have official languages that are taught in schools, likely before German/Hungarian. At minimum, I'd expect Czech in Bohemia-Moravia, Polish and Ruthenian(Ukrainian) in Galicia-Lodomeria, Croatian in Croatia and Dalmatia (and possibly Istria and Bosnia), Serbian in the Banate (and possibly Bosnia), Romanian in Transylvania, Friulian in Friuli/Gorizia, Italian (or Venetan if they're trying to break links to Italy) in Trieste and Dalmatia, Slovak in OTL Slovakia (sooner or later Budapest is going to have to either grant it some degree of autonomy or else Vienna will split it off). Might also have some smaller groups like Rusyns, Romani, Aromanians etc get lucky and get some degree of official recognition.
Most likely, you end up with a system of mother-tongue education in primary school, National-language education in middle school, and German/Hungarian education in high school and university. The average citizen would be at least bilingual (Germans and Hungarians are likely to have the smallest linguistic repetoirs), while being able to speak 3 or 4 languages would be fairly unremarkable. Common foreign languages that might be taught as elective subjects could include Italian, Russian, French, English and Turkish, as well as the other national languages of the Empire.
(Gods, the Reichstag is likely to be as reliant on interpreters as the European Parliament is).